Statins, inhibitors of HMGCoA reductase, are widely used to control cholesterol levels. Exciting epidemiological studies are beginning to reveal that long-term use of statins might have a protective effect on some types of cancers. Over the previous cycle of this application we demonstrated that statins inhibit cyclin-dependent-kinase 2 (cdk2) not only by inducing the cdk inhibitor p21 in HCT116 colon cancer cells, but also by the unexpected mechanism of inhibiting the phosphorylation on threonine 160 of cdk2 in prostate cancer PC3 cells. Phosphorylation at this site is essential for cdk2 kinase activity. We also demonstrated that statins inhibit G1-S progression in a wide range of cells through inhibiting the geranylgeranylation of a key G protein. We will now dissect the mechanism by which HMGCoA reductase inhibition and inhibition of geranyl- geranylation leads to inhibition of T160 phosphorylation and cancer cell inhibition. In the first Aim we will test the hypothesis that a critical Rho/Rab type G protein needs to be geranyl-geranylated to activate the signal transduction pathways that are essential for phosphorylation and activity of cdk2. In the second Aim we will test the hypothesis that activity of the signal transducing serine-threonine kinases downstream from the Rho/Rab G proteins are critical for activation of cdk2 phosphorylation. In the third Aim we will use specially engineered cells (and cell growth conditions) to determine exactly which node in the signal transduction pathway downstream from HMGCoA reductase inhibition is critical for G1-S cell-cycle progression and for the proliferation of cancer cells in culture and in xenografts. Overall this project will dissect exactly how HMG CoA reductase inhibition inhibits cyclin-dependent kinases and G1-S progression and will critically test whether inhibition of G1-S progression is the reason why statins inhibit proliferation of cancer cells in vitro and in vivo. The results will have major implications for the use of statins as anti-cell-cycle and anti-cancer agents and will suggest ways to improve these effects of statins through the additional inhibition of other signal transducers. There are very few agents available to prevent the onset of cancers. Statins are extremely safe in humans and widely used clinically for lowering cholesterol levels. Thus, if they prove to have potent anti-cancer effects, it will be easy to adapt them for prevention and therapy of cancers in humans. On the other hand, although we know many details of the cell-cycle, very few chemical agents are available that are as safe as statins and block cell-cycle progression as effectively. This project will join these two lines of research by exploring how statins inhibit cell-cycle progression and testing whether such inhibition is important for the anti-cancer effect of statins. Results from this project will promote the rational use of statins as anti-cell-cycle and anti-cancer agents and also suggest the use of other chemicals that will synergize with statins in their anti-cancer effect.